Tools which a user grips for support or operation are part of most people's every day recreational and work life. From ski poles to manual and power tool handgrips, all require maximum hand control for safety and performance.
NASA anthropometric measurements show that there are substantial variations in hand breadth within gender as well as between genders. In assuming grip positions, the power grasp is the grip used in applications such as holding a power drill, carrying a pipe, or grasping a ski pole where the thumb is generally in direct opposition to the fingers, and it is the maximum force that can be exerted by the hand. The oblique grasp is the grip used with tools such as wrenches, screwdrivers, and paint scrapers, and it can utilize only about 65 percent of the strength of the power grip. Both grip types demand different handgrip lengths consistent with hand breadth variations for maximum comfort and effectiveness. A continuous muscular stress greater than approximately 15% of the maximum stress of a muscle group results in early fatigue of that muscle group with attendant compromise of task effectiveness, increased musculoskeletal disorders, and accident rates. (W. Rohmert, Applied Ergonomics, Vol. 4, pages 91-95, 1973; Butterworth Scientific Ltd.) From skiers gripping ski poles to workers operating various types of hand and power tools, it is important for those users to maximize control while exerting minimal muscular effort and thereby reducing fatigue.
The upper bearing surface of the hand is the thumb and first digit or radial segment of the hand and the lower bearing surface of the hand is the outside edge or ulnar portion of the hand. When a hand grasps a handgrip, the palmar region normally presses against the handgrip body to control the implement or tool. There is often insufficient purchase depending on handgrip length and diameter, and inadequate support on the upper and lower bearing surfaces of the hand. Cradling the upper and lower bearing surfaces of the hand when gripping a handgrip substantially increases purchase and therefore increases control and safety while reducing the strength levels needed to successfully complete a task.
There have been several inventions that have focused on segmented handgrips that will, in effect, partially conform to a user's grip by adding spacers for grip diameter adjustment or by adding upper and/or lower projections or surfaces.
One example of a handgrip with a variable shape is U.S. Pat. No. 4,645,235 which describes a multi-element ski pole handgrip, one whose elements can be changed to offer different handgrip thicknesses. Other examples of designs to improve handgrip comfort and control are U.S. Pat. Nos. 4,750,760, 3,992,021, 3,879,048, and 3,436,090 which describe ski pole handgrips contoured with upper and lower support surfaces.
The aforementioned patents are restricted to a particular handgrip length which corresponds to a specific hand breadth. It is critical, however, for the hand to fit snugly with the handgrip because a smaller hand relative to a handgrip will allow unwanted hand movement relative to the handgrip, and a larger hand relative to a handgrip will compromise grip strength and comfort because of an incorrect fit. Both result in decreased task effectiveness and safety. Having the ability to adjust the cradling capability of a handgrip maximizes the contact surface area on the gripping hand and allows minimal movement of the hand relative to the handgrip. By applying this support, less effort is required on the part of the user to effect maximum control with substantially less fatigue. This functional differentiation will result in improved performance, comfort, and safety.
There is a need for the present invention because none of the aforementioned inventions address the problem of adjusting handgrips to the user's hand breadth.